I've just put up a quick tutorial on using the High Pass Filter and the Overlay blend mode to sharpen your images in photoshop.

I suggest reading it on my site to see the sample and before/after images, but I'll include the text here as well.

Quote:

The high pass filter is a great way to give your images a little extra pop. The name might sound strange, but you’ve probably already seen it at work. Gatorade uses it a lot, or at least they used to. It works nicely for certain types of cosplay shots, and in this tutorial I’ll show you one way to create the effect.

Essentially, it’s contrast adjustment on a very small, local scale. Where ever dark and light pixels meet, the dark pixels get darker, and the light ones get lighter. This makes visible edges stand out more, creating the illusion of a sharper image. I say “illusion”, because no amount of high pass filter/unsharp masking will fix a blurry photo. The filter we’re applying isn’t intelligent; it doesn’t know or care about what “focus” is. All it cares about is lightening light pixels and darkening dark pixels.

Generally, I find that edge sharpening works best on heroic type costumes, or action scenes. Superheroes are a great choice for this kind of effect. Princesses are less great. The high pass filter will cause marks on your model’s skin (pores, acne, scars, etc.) to stand out more. Whether or not that’s a good thing or a bad thing is up to you.

Step 1. Start by opening the file you want to work on in Photoshop. For this example, I’ve picked a pretty awesome WarMachine cosplay from FanExpo 2011. There are lots of hard lines and details in this costume, and the high pass filter is going to help pick those out.

Step 2. Create a duplicate layer. Go to Layer > Duplicate Layer…

Step 3. We want this new layer to be greyscale, so with the duplicate layer selected, go to Image > Adjustments > Desaturate. At this point, you’ll have a black and white version on top of the original version of your photo.

Step 4. Now, we apply the high pass filter. Go to Filter > Other > High Pass. A window will pop up with a radius slider. This will control how strong the final effect is. A smaller radius will generate a more subtle sharpening than a large radius. I find that ~75 is a good number for me, but if you’re using a lower resolution photo, you’re going to want to lower the slider accordingly. You might need to experiment a bit with this step to find out what works for you. Remember, ctrl+z is your friend!

Step 5. This is where the magic happens. It’s time to change the blend mode of our black and white layer. From the blend mode drop down menu select Overlay. Most of the blend mode settings in this area will work (Overlay, Soft Light, Hard Light, etc.) but will give you different effects. If you’re lost, you’ll find the blend mode dropdown in the layer palette. It probably says “normal”.

Step 6. Tone it down. The effects of the high pass filter are pretty strong, and I always want to ease off for a more natural looking effect. “Less is more” is a good thing to keep in mind, whenever you’re doing any kind of post processing.

By adjusting the opacity of the top layer, you can pull the effect back a little. Again, you’ll probably want to play around with this step, but I like to keep it around 30% opacity or so.

And that’s it! The finished product. You might be saying “hey… that looks exactly like the original”, so I created a these before and after slides to illustrate the difference. See how much sharper the lines look? That’s the power of the high pass filter!

For those interested, this effect is very similar to what Clarity does in Adobe RAW or LightRoom. It works great for grunge inspired photos, photos of mechanical works, anything that you want a harsh and gritty effect. It does not work so great for softer photos, like female portraits where you want sharpening but don't want the lighting to appear harsh and don't want to highlight all of their blemishes. I eventually stopped using this method and just started using filters/sharpening/unsharp mask. Unsharp mask will enable you to sharpen edges, like eyes and hair, without detailing every blemish and wrinkle on a persons skin. Of course, if that's what you're aiming for, then unsharp mask can do it as well. I only write this because if you've been using this method, you might want to try out unsharp mask for all of your softer works.